I actually don't think anyone is allowed to sell cotton nightgowns, by Federal Regulation. Back in the 70s, I think, some children burned in their PJs and died so they made a law about children's sleepwear. I just googled it and found the rules:
http://www.cpsc.gov/businfo/regsumsleepwear.pdf
The sleepwear (sizes 9 months to child's 14) has to either be flame resistant (poly) OR tight-fitting (which can be cotton). So we go the tight-fitting route in order to wear cotton. The "tight fitting" ones look like leggings and a long-sleeved tee. Or for summer they look like bike shorts and a short-sleeved tee. And when my daughter is really wanting a nightgown, it's got to be poly, because should a fire break out, the looseness makes it easier to catch fire and the air between the PJ and skin actually causes the fire to grow rather than extinguish.
Here's a website that explains it as well:
http://www.pediatricsafety.net/2009/10/childrens-pajamas-...
And here:
http://www.ehow.com/facts_###-###-####_law-flame_resistan...
And here:
http://wallypop.wordpress.com/2011/10/28/flame-retardant-...
Bummer, because I love the look of cotton nightgowns. But I get the need for the precaution. I remember being young and hearing about some burned kids in cotton nightgowns, so it's stuck in my head.
Hope that helps!